1. Field of the Invention
This invention relates in general to devices for treating exhaust gases and, in particular, to a new and useful catalytic converter for the treatment of exhaust gases of internal combustion engines which includes a monolithic ceramic honeycomb structure supported within a housing in and a flow passage for the gases by a compacted mesh material disposed between the honeycomb and the housing and holding the honeycomb in position.
2. Description of the Prior Art
The present invention is particularly concerned with the construction of catalyst converters which are connected to the exhaust conduits of internal combustion engines in order to reduce the harmful or polluting substances contained in the exhaust gases. The polluting substances particularly include carbon monoxide, unburnt or partly burnt hydrocarbons (CxHy) as well as nitric oxides. It is known that catalysts can be used with advantage to convert these harmful substances into unharmful emissions. The known exhaust gas catalysts include ceramic porous carriers having surfaces over which a catalytically active substance such as platinum is applied and such carriers usually take the form of small cylindrical or spherical pellets. These pellets, usually in dimensions of between 1 and 4 mm in diameter, are embedded in a sheet metal housing provided with perforated sheets which are traversed by the exhaust gas from the engine.
The main disadvantages of the known pellet type catalysts are:
1. The high flow resistance which they produce along with the associated high exhaust gas pressure and this in turn, results in a drop of the maximum attainable engine performance;
2. The relatively high heat capacity of the catalyst which prevents rapid heating to operating temperatures after a cold start of the engine; and
3. The great abrasion of the pellets which materially shortens the life of the known detoxification devices.
It is also known that monolithic carriers which consist of a continuous skeleton of porous ceramic material can be used with advantage. Such elements, which are called honeycombs in the industry, comprise cylindrical blocks of about 100 mm in diameter and are of lengths between 50 and 200 mm. Such dimensions are typical of exhaust gas catalytic systems which are used in European medium sized passenger cars.
The flow channels formed by the skeleton are usually of square or trapezoidal cross section with an inside cross section from between 1 and 32 mm. Thus, several thousand flow channels with substantially constant cross sections are reached in a body of the above-described size. Because of the low wall thickness of a few tenths of a millimeter of their rib structure, the honeycombs have relatively low mechanical strength and are susceptible to mechanical stress by rapidly changing temperatures, particularly thermal shock. The coefficient of thermal expansion is substantially lower than that of the metal used as a holder. For this reason direct mounting of the honeycomb in a rigid sheet metal construction is out of the question. Attempts to embed the honeycomb in a metal housing with an intermediate layer of high temperature resistant ceramic fiber have been made, but the results were not satisfactory. The great pulsation of the exhaust gases with dynamic alternating pressure of several tenths of atmospheres in cooperation with the high temperature gases of 800.degree. C. or more, destroys the ceramic fiber within a few operating hours.